Oscillator frequency control



Oct. 5, 1954 R. F. SCHWARTZ OSCILLATOR FREQUENCY CONTROL 2 Sheets-Sheet1 Filed Oct. 15,. 1953 Pan/.5e /4 l SI/PPL Y l wafer/0M fsw/eci f lV INl E N T0 R. /P/'carld E Schwartz a wa a@ m M# w l D f/.v L ma M065 5 3l@ @N g 1,5 ZNU M/ j@ 5 uw X796 3M Il /5. .f ,f er ,km 4 EL wn f MW .mps www 2 Oct. 5, i954l R. F. SCHWARTZ OSCILLATOR FREQUENCY CONTROL FiledOct. 13, 1953 SUP/"l Y sauf@ W 2 Sheets-Sheet 2 Patented Oct. 5, 1954ITED STAT f. S

TENT OFFICE SCHLATOR FREQUENCY CONTROL Richard F. Schwartz,Philadelphia, Pa., assignor to Radio Corporation of America, acorporation of Delaware 12 Claims. 1

This invention relates to the frequency control of oscillationgenerators, and more particularly to the frequency control orstabilization of ultrahigh frequency (UHF) or microwave oscillators,such as magnetrons.

This invention is related to and constitutes an improvement over thatdisclosed in the copending Koresv application, Serial No. 177,455, ledAugust 3, 1950. Said application discloses arrangements for thefrequency stabilization of magnetrons by means of the so-calledinjection locking process, in which a small amount of power from astable frequency source (the frequency of which is harmonically relatedto the magnetron frequency) is injected into a magnetron oscillator inorder to lock the frequency of the high-power magnetron. In theaforementioned Koros system, it has been found under conditions ofamplitude or anode modulation, that the input impedance presented by themagnetron to the injection power source varies during the modulationcycle. This means that there is a change of loading on the injectionsource during the modulation cycle, resulting in a change in theeffective injection power applied to the magnetron during the modulationcycle. This is disadvantageous since it reduces the effectiveness or theefliciency of the frequency stabilization process.

Therefore, an object of this invention is to devise for oscillators aninjection locking system which functions to maintain the injection powerapplied to the oscillator substantially constant, irrespective ofchanges in the impedance presented to the injection power source by theoscillator.

Another object is to provide an injection locking system for modulatedmagnetron oscillators which operates 'to maintain the injection powerapplied to the magnetron substantially constant throughout themodulation cycle.

The objects of this invention are accomplished, briefly, in thefollowing manner: two sources of stable frequency injection power areutilized, coupled at spaced points to the main transmission line (theline between the magnetron oscillator and its load). The combination ofthe two sources gives a virtual injection point that can shift, if themagnetron requires it. Alternatively, a single injection source can beutilized, coupled to the main transmission line at two spaced points bymeans of separate branch transmission lines.

The foregoing and other objects of this invention will appear from areading of the following description of some exemplications thereof,reference being had to the accompanying drawings,

wherein:

Fig. 1 is a diagrammatic representation of a known injection lockingsystem;

Fig. 2 is a similar diagram of a system according to the invention;

Fig. 3 is a diagram of a modified system, using only one injectionsource; and

Fig. 4 is a similar diagram of a modification of Fig. 3.

Referring to Fig. 1, magnetron I has a conventional cathode 2 (the outershell or anode of the magnetron being grounded as shown) which isconnected through an amplitude modulator 3 to the negative terminal of ahigh voltage unidirectional pcwcr supply li the positive terminal ofwhich is grounded as indicated. Thus, magnetron l is energized togenerate oscillatory energy at a frequency determined mainly by theparameters of magnetron I. A modulating signal, such as a televisionvideo signal for example, is fed from a signal source through a couplingcapacitor 5 to the control grid :5 of a vacuum tube l, for example apentode as shown, which constitutes amplitude modulator 3. The anode 8of pentode l is connected directly to magnetron cathode 2, while thecathode 9 of said pentode is connected directly to the negative terminalof power supply 4. Thus, tube l of modulator 3 is connected in series inthe cathode circuit of magnetron I, so that such magnetron may beconsidered to be anode-modulated. Because the cathode, of necessity, isin the anode circuit, the modulator 3 is, in effect, in theanode-cathode circuit of magnetron I.

Magnetron I is provided with the usual output or load coupling loop II)by means of which oscila latory energy is abstracted from such magnetronand fed by means of a main transmission line I I to a load I2 which maybe, for example, a transmitting antenna. Line II is indicatedschematically as including only a single conductor, but this line ispreferably a coaxial line, or it may be a waveguide.

A branch transmission line I3, which is indicated schematically asincluding only a single conductor but which may be a coaxial line or awaveguide, is coupled to the main line I I at junction point B, in orderto effect injection of stable frequency power into the main line andinto magnetron I. An injection source I4, illustrated inside thedotted-line box in Fig. l, is coupled to that end of branch line I3opposite to point B. Source I4 feeds injection power into main line IIand magnetron I by means of branch line I3. Source I4 may include, forpurposes of illustration, a grounded-grid amplifier tube I5 having atuned output circuit I6 which is inductively coupled to branchtransmission line r3 and is tuned to a frequency harmonically related tothe desired frequency of operation of magnetron oscillator I. Amplifiertube I5 is driven from a suitable stable frequency driving source, suchas a crystal oscillator, by means of a coupling including leads I 'Iwhich inductively couple oscillatory energy to the inductive portion ofan LC network I8 connected to the cathode I9 of tube I5.

Fig. l illustrates an amplitude modulated magnetron oscillator which isinjection locked by injection source Hl, in order to stabilize itsfrequency of operation. In other words, injection power is fed into themagnetron I to eect stabilization of its output frequency in response tothe output of a stable-frequency crystal oscillator. Fig. 1 discloses asystem of the prior art, as exemplified by the aforementioned Korosapplication, and for further details concerning the operation thereof,reference may be had to the said copending application.

In Fig. l, curve I represents the injection voltage distribution on aportion of the main transmission line i i, while curve II represents theinjection voltage distribution on the branch line i3. The voltagedistribution thus represented is the voltage produced by the injectionsource if?. acting as a generator. Point A is a zero or voltage minimumpoint of the standing wave I resulting from the injection power appliedto the magnetron. It has been found, by means of measurements made onthe system of l? ig. i, that the magnitude of the input impedance of themagnetron I to the injected signal (that is, the impedance presented bythe magnetron to the injection source generator) varies during themodulation cycle. The magnetron I is the terna'- nation of thetransmission line from the injec tion source Hi, considered as agenerator, and therefore the magnetron influences the position and alsothe magnitude of the voltage minimum along the line I I. As themagnitude of the magnetron input impedance varies (during the modulationcycle) the magnitude and position of the minimum in the voltage standingwave generated by the injection source must likewise change. Thus, pointA shifts during the modulation cycle. For example, in one case measured(using the system of Fig. l) it was found that the input impedance ofthe magnetron at the crest of the modulation cycle was 23.3/- fi.3 ohms,while at the trough of the modulation cycle it was lle/dai? ohms. Thiscor esponds to a shift in point A of .0325 wavelength. This shift,though small, causes a change in the loading of the injection source,resulting in a change in the effective injection power applied to themagnetron during the modulation cycle. This change in injection power isundesirable, and is reduced substantially by the arrangement of thisinvention.

Moreover, points A and B (the latter of which, it will be remembered, isthe junction point between the main transmission line II and the branchtransmission line i3) do not ordinarily coincide. In the aforementionedtested case, neither of the positions of point A (that is, neither itsposition at the crest of the modulation cycle nor its position at thetrough of the modulation cycle) corresponded to the position of point B.

Fig. 2 discloses an arrangement according to this invention. In thisiigure, elements the same as those of Fig. l are denoted by the samereference numerals. In Fig. 2, two separate injection sources I4 and I4are utilized to feed injection power into the magnetron i. Each of thesesources may, if desired, be exactly similar to injection source Ill inFig. l, and the two injection sources are both driven from a commoncrystalcontrolled driving source 20, for example a crystal-controlledfrequency multiplier chain.

The output of injection source I4 is applied to the magnetron i by meansof a branch transmission line i3 connected to source ifi and joined tothe main line i! at point B, while the output of injection source ill isapplied to magnetron I by means of a separate branch transmission linei3 connected to source I4 and joined to the main line i I at point B.Points B and B are located some distance apart, for example aquarter-wavelength at the frequency of magnetron oscillator I. Also,according to this invention, points B and B are so located that point A(the aero or voltage minimum point of the standing wave I resulting fromthe injection power applied to the magnetron) is between points B and B.In Fig. 2, curve II represents the injection voltage standing wavedistribution on branch line i3, while curve III represents the injectionvoltage standing wave distribution on branch line I3.

In Fig. 2, the two injection sources if; and ifi', which are operatingat the same frequency since they are both driven by the same drivingsource Eil, both combine to ei'lect injection locking (and thereby alsofrequency stabilization) of the magnetron I. If there is any tendencyfor the input impedance of the magnetron to vary during the modulationcycle, the voltage minimum point A will tend to shift in position, aspreviously described in connection with Fig. l. As the voltage minimumpoint A of the injection voltage standing wave I shifts, each of theinjection sources ld and Ill sees a changed impedance. Therefore, unlessthese sources have zero internal ini-- pedance (which they ordinarily donot) they must supply either more or less volt-amperes, depending uponhow their loadings have changed. Since point A is between points B andB', as this point shifts it approaches nearer one of the branch linejunctions B or B and rececles further from the other such junction;thus, the loadings of the two sources It and ifi' move in oppositedirections as point A shifts, and the source whose branch line junctionis closest to the voltage minimum A will see the lower impedance whilethe other source will see the higher impedance. Since the loadings ofthe two injection sources lll and Ill thus move in opposite directionsas point A shifts during the modulation cycle, one such source will tendto deliver fewer voltamperes to the magnetron, whereas the other sourcewill tend to deliver more volt-amperes. Hence, the volt-amperesavailable for injection locking will be much more nearly constant duringthe modulation cycle with the locking system of Fig. 2 than with thelocking system of Fig. l.

The action of the Fig. 2 system may be expressed in another way. Thecombination of the two injection sources Eli and i4 gives a virtualinjection point that can shift, if required to by changes in themagnetron input impedance.

Fig. 3 is a modified system. In Fig. 3, only a single injection sourcei@ is utilized, but this source feeds injection power to the maintransmission line Il (and thereby also to the magnetron i) by means of abranch transmission line constituted by two separate arms 2l and 2Iwhich are joined to the main line at two spaced points B and B. Points Band. B' are again located some distance apart, and point A (the minimumvoltage point of the standing wave I resulting from the injection powerapplied to the magnetron) is between points B and B. The distances alongthe two arms 2E and 2|', from the respective junctions B and B to theinjection source iii, are equal to each other.

The action in Fig. 3 is quite similar to that in Fig. 2. As the voltageminimum point A shifts or tends to shift, a changed impedance will bepresented to each of the arms 2l and 2l. The impedance changes thuspresented to the two arms are in opposite directions. Since theimpedances presented to the two arms 2| and 2i thus change in oppositedirections as point A shifts during the modulation cycle, one such armwill tend to carry fewer volt-amperes to the magnetron, whereas theother arm will tend to carry more volt-amperes. Thus, the virtualinjection point resulting from the combination of the two injectionpoints B and B shifts as point A shifts, giving a very nearly constantinjection locking power throughout the modulation cycle.

Fig. 4 is a modification of Fig. 3. In Figs. 1, 2 and 3, amplitudemodulation of the magnetron is effected. However, in Fig. 4 angularmodulation of the magnetron, with stabilization of the mean frequency,is carried out. In Fig. 4, the magnetron cathode 2 is connected directlyto the negative terminal of power supply 4, and no amplitude modulatoris utilized. A single injection source I4 is utilized, feeding power tothe main transmission line Il by means of two separate arms 2l and 2ijoined to the main line at two spaced points B and B', exactly as inFig. 3.

In Fig. 4., the injection source, instead of being driven by acrystal-controlled (fixed-frequency) driving source, is driven by anarrangement which includes a means for producing angular modulation ofthe locking source. For example, and as illustrated, the means forproducing angular modulation of the locking source may consist of afrequency modulated source driver 22 to which a modulating signal isfed, which source driver 22 includes therein any suitable means forstabilizing the center (or rest) frequency of the same. Since theinjection source is thus angularly modulated by its driving source 22,the magnetron will follow the instantaneous angle of the injectionvoltage. center (rest or unmodulated) frequency of the magnetron isstabilized due to the injection locking action of injection source I4.

Alternatively, the means for producing angular modulation of the lockingsource may consist of a crystal controlled frequency multiplier chain(for driving the injection source) which is phase modulated at someintermediate point. Any other angle modulated source will work equallywell.

II'he present invention is not limited to oscillators of the magnetrontype. It can be applied to oscillators of any type capable of beinglocked in frequency.

What is claimed is:

1. A frequency control arrangement comprising an oscillator whosefrequency is to be controlled, a transmission line coupling the outputof said oscillator to a load, and means for coupling injection power ofstable frequency into said line at two separate spaced points thereon,said points being so positioned that a voltage minimum of the standingwave pattern set up by said injection power is located between said twopoints.

At the same time, the u 2. An arrangement as dened in claim 1, whereinthe oscillator is a magnetron oscillator.

3. An arrangement as defined in claim 1, wherein the said meanscomprises two sources of injection power the outputs of which arecoupled to said transmission line at spaced points thereon, by means ofseparate respective output couplings.

4. An arrangement as dened in claim 1, wherein the said means comprisesa source of injection power the output of which is coupled to saidtransmission line at spaced points thereon, by means of two separatecouplings.

5. An arrangement as dened in claim 1, wherein the oscillator is amagnetron oscillator and wherein the said means comprises two sources ofinjection power the outputs of which are coupled to said transmissionline at spaced points thereon, by means of separate respective outputcouplings.

6. An arrangement as defined in claim 1, wherein the oscillator is amagnetron oscillator and wherein the said means comprises a source ofinjection power the output of which is coupled to said transmission lineat spaced points thereon, by means of two separate couplings.

7. A frequency control arrangement comprising a diode cavity-typeoscillator whose frequency is to be controlled, means for amplitudemodulating the output of said oscillator in accordance with a modulatingsignal, a transmission line coupling the output of said oscillator to aload, and means for coupling injection power of stable frequency intosaid line at two separate spaced points thereon, said points being sopositioned that a voltage minimum of the standing wave pattern set up bysaid injection power is located between said two points.

8. An arrangement as defined in claim 7, wherein the oscillator is amagnetron oscillator.

9. An arrangement as defined in claim 7,wherein the means for couplinginjection power comprises two sources of injection power the outputs ofwhich are coupled to said transmission line at spaced points thereon, bymeans of separate respective output couplings.

10. An arrangement as defined in claim 7, wherein the means for couplinginjection power comprises a source of injection power the output ofwhich is coupled to said transmission line at spaced points thereon, bymeans of two separate couplings.

11. An arrangement as defined in claim 7, wherein the oscillator is amagnetron oscillator and wherein the means for coupling injection powercomprises two sources of injection power the outputs of which arecoupled to said transmission line at spaced points thereon, by means ofseparate respective output couplings.

12. An arrangement as defined in claim 7, wherein the oscillator is amagnetron oscillator and wherein the means for coupling injection powercomprises a source of injection power the output of which is coupled tosaid transmission line at spaced points thereon, by means of twoseparate couplings.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,620,467 Donal Dec. 2, 1952 2,677,058 Kirkman Apr. 27, 1954

